Prior to the present invention, weld-bonding was limited primarily to aluminum alloy structures using an epoxy adhesive resin. Advancing technological requirements rendered weld-bonded aluminum alloy structures using epoxy adhesives unsuitable for temperatures beyond 450 K maximum. Thus, weld-bonding of titanium metal using a high temperature adhesive system are the materials currently used when the temperatures exceed 450 K.
High temperature adhesives, i.e. 500 to 650 K, which could be used for the weld-bonding applications are polyimides, polypyrrones, polybenziamidazole, polyquinoxalines, polyphenylquinoxalines, or polyimidazoquinazolines. Several problems arise from the use of these adhesives in conjunction with titanium weld-bonding techniques. All of the high temperature polymers which have been considered for adhesive systems are condensation cured and evolve gaseous products during the cure. The high void content of these adhesives may be overcome by applying pressure during cure of the adhesive. However, where the workpiece is extremely large, application of pressure to collapse the voids and extrude a small portion of resin from the bond joint may require a very large machine or may be impossible. Thus, adhesives which evolve only a small amount of gaseous by-products upon curing are preferred.
Because of the poor oxidative stability of titanium in air, the adhesive resin used in weld-bonding must be applied to the titanium surface within four hours after cleaning of the surface. This presents a problem for the fabricator in that he must clean the titanium surface to be bonded, apply the resin adhesive, and proceed with the weld-bonding before the adhesive resin becomes oxidized from the air. Thus, several fabrication steps are necessary within a specified time confinement.
U.S. Pat. No. 3,337,711 teaches a prior art method of weld-bonding two pieces of metal using a thermally, softenable adhesive. The patentee shunts the current between the metal layers to be bonded, causing the metal layers to heat and soften the adhesive resin. This method of weld-bonding is not applicable to titanium because of titanium oxidative instability. When titanium is welded, care is taken to prevent excessive heating of the titanium workpiece, and this is usually done by using refrigerated electrodes.